<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <style>
        div {
            width: 200px;
            height: 100px;
        }
        .one {
            width: 500px;
            height: 200px;
            background-color: red;
        }
        .two {
            width: 200px;
            height: 100px;
            background-color: green;

            margin: 0 auto;
        }
    </style>

    <!-- <div class="one">
        这是一个元素
    </div>

    <div class="two">
        这是一个元素
    </div> -->
    <div class="one">
        <div class="two">
            hello
        </div>
    </div>
</body>
</html>